1. Field of the Invention
The present invention relates to systems for driving plural or multiple lamps, and particularly to a system typically used for driving plural or multiple lamps of a backlight module of a liquid crystal display (LCD).
2. Related Art
Generally, a discharge lamp used as a light source of a liquid crystal display (LCD) panel has a terminal voltage characteristic that varies according to the frequency of alternating current applied to the discharge lamp. The discharge lamp also exhibits a negative impedance characteristic whereby the impedance decreases upon an increase in input power. A circuit for driving the discharge lamp can for example be an inverter. The circuit should be configured with a controllable alternating current power supply and a feedback loop, for monitoring the current flowing through the lamp to ensure stable operation and to regulate the load as well.
A conventional system for driving plural lamps normally requires two pulse width modulation (PWM) controllers. FIG. 5 is a block diagram of a conventional system for driving plural lamps. The system comprises a converter 1, a bridge driving circuit 2, a lamp module 3, a PWM controller 4, and a bridge controller 5. The converter 1 converts a received direct current (DC) voltage or alternating current (AC) voltage to a DC voltage. The bridge driving circuit 2 converts the DC voltage received from the converter 1 to an AC voltage. The AC voltage is used for igniting lamps in the lamp module 3. The PWM controller 4 receives the DC voltage outputted from the converter 1, and generates a feedback signal to control the converter 1 to output a fixed DC voltage. The bridge controller 5 is another PWM controller, which is connected between the lamp module 3 and the bridge driving circuit 2. The bridge controller 5 is used for controlling the bridge driving circuit 2, in order to regulate current flowing through the lamp module 3.
Thus, the conventional system for driving plural lamps uses two PWM controllers. The two PWM controllers respectively receive different feedback signals to control the system. One is used for regulating current flowing through the lamps, and the other is used for fixing the DC voltage outputted from the converter 1. A PWM controller is expensive, especially a PWM controller used for driving a multi-lamp module. Therefore, the cost of the conventional driving system requiring two of the PWM controllers is inevitably high.
It is desired to provide a system for driving plural or multiple lamps such as those of an LCD, the system having a reduced number of PWM controllers and a relatively low cost.